Ok, well it's not so much a 1-2-3, more a case of following your nose and picking useful bits from other projects.
Here's the simplest example, using a single, regular LED:
https://projects.drogon.net/raspberry-p ... ingle-led/
But your project has a higher current and higher voltage.
Here's an example using a relay. The Pi can not drive a relay directly, it needs a buffer transistor. Depending on your relay you can control much bigger loads:
http://www.geekingabout.com/controlling ... -gpio.html
But, if you use a solid-state driver you can turn the LEDs on and off rapidly, which allows you to control their brightness:
In that example he is controlling three strips (red, green and blue) so he has three power transistors. You only need one as you are only driving one strip.
Oh, and you should find that most people here are helpful and happy to help, so ask any question.